The present invention generally relates to communications systems and, more particularly, to a receiver.
In some communications systems it is desirable to detect the presence of a co-channel interfering signal and remove it. For example, during the transition from analog to digital terrestrial television in the United States, both analog NTSC (National Television Systems Committee) based transmissions and digital ATSC-HDTV (Advanced Television Systems Committee-High Definition Television) based transmissions are expected to co-exist for a number of years. As such, an NTSC broadcast signal and an ATSC broadcast signal may share the same 6 MHz wide (millions of hertz) channel. This is illustrated in FIG. 1, which shows the relative spectral positions of the NTSC signal carriers (video, audio and chroma) with respect to the digital VSB (Vestigial Sideband) ATSC signal spectrum. Thus, an ATSC receiver must be able to efficiently detect and reject NTSC co-channel interference.
In an ATSC-HDTV digital receiver, NTSC co-channel interference rejection may be performed by the comb filter (e.g., see, United States Advanced Television Systems Committee, “ATSC Digital Television Standard”, Document A/53, Sep. 16, 1995). The comb filter is a 12 symbol linear feed-forward filter with spectral nulls at or near the NTSC signal carriers, and is only applied when NTSC interference is detected (e.g., see, United States Advanced Television Systems Committee, “Guide to the Use of the ATSC Digital Television Standard”, Document A/54, Oct. 4, 1995). Tests have shown that the comb filter performs efficient NTSC signal rejection for D/U (Desired-to-Undesired) signal power ratios up to 16 dB (decibels). The D/U signal power ratio is defined as the average digital VSB ATSC signal power divided by the peak NTSC signal power.
As such, the method of removing NTSC co-channel interference is typically to leave the comb filter enabled in the data path and compensate for its presence in a convolutional decoder of the receiver. Unfortunately, this adds much complexity and cost to the hardware implementation.